This invention is directed to a gas-tight, sealed alkaline secondary cell with positive and negative electrodes, an intermediate separator, and a layered auxiliary electrode that is electron-conductively connected with the negative electrode for reducing oxygen pressure. The present invention finds applicability to all types of gas-tight alkaline cells which operate on an oxygen-consumption cycle, especially Ni/Cd, Ni/Zn and Ni/hydride storage batteries.
As is known, an accumulation of oxygen generally results when the cells of this type are overcharged. However, this can be effectively prevented by "enlarging" the surface of the negative electrode, through electrical connection with an appropriate auxiliary electrode. In so doing, the oxygen-consumption reaction EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4 OH.sup.-
is kinetically hindered to a lesser extent on the auxiliary electrode than on the negative (main) electrode, as is desired. To this end, the negative auxiliary electrode may contain any of a number of catalysts, including metals such as platinum, silver and copper, or carbon, and is preferably highly porous to develop a large inner surface.
The amount of electrolyte used is of critical consequence to oxygen consumption since, on the one hand, only a small amount of electrolyte should be used for preventing overcharging, while on the other hand, loading of the cell with high currents requires a large amount of electrolyte. To provide a proper balance, the amount of electrolyte should be adjusted to establish the three-phrase equilibrium which is needed on the active electrode surface for the conversion of oxygen. As distinguished from gas-tight Ni/Cd cells, in Ni/hydride cells a complete collection of oxygen gas is made particularly important for this reason, because the active alloy corrodes in its presence.
In adapting to the generally relatively narrow electrode structure of storage batteries exhibiting high energy density and power density, the auxiliary electrodes (as are the electrode plates) are thin, layer-like forms which are often closely adjacent to the negative plates in the stack of plates comprising the cell.
DE-AS 15 96 223 discloses a Ni/Cd storage battery with an auxiliary electrode, formed as a coiled cell in which the auxiliary electrode covers the final spiral of the negative electrode and at the same time is in close contact with the inner wall of the housing which forms the negative cell pole. This cover layer on the negative electrode contains finely divided silver (or some other metal more electropositive than cadmium) in a porous, hydrophobic carrier material so that when in contact with the negative electrode, a negative mixed electrode is formed on which the rate of oxygen reduction is increased.
DE-PS 28 38 857 discloses a metal oxide/lanthanum nickel hydride storage battery having an oxygen-reduction electrode comprised of a nickel network coated with carbon black. The nickel network is short-circuited (wired) with the negative electrode and is covered near the positive electrode with a separator which possesses an affinity for electrolyte which is less than that of the two main electrodes, so that the flow of oxygen is only slightly hindered. In contrast, the hydride electrodes are to the extent possible protected against diffusing oxygen by insulation with a second separator material which, in the wet state, possesses a relatively low gas permeability.